Evaluation of Fabrication Errors on the Performance of Injection Molded Light Pipes (Waveguides)

Abstract

This paper examines the consequences of fabrication effects on the performance of injection molded plastic light pipes for distributive automotive lighting. It discusses the magnitude of these effects on propagating and output light. Molding errors, such as sinks, voids, flow lines, knit lines, and warpage, will be examined through computer modeling and measurement of sample distributive lighting systems. Two example light pipe systems will be presented, an instrument cluster pointer system and a regional distributive interior lighting system. For the pointer system, computer modeling results, such as total light output and output exitance distributions will be presented. Modeling results will be presented for both an error free system and system that include the aforementioned molding errors. These results will be compared to determine the output light loss and redistribution. For the regional interior lighting system, total output and exitance measurements of fabricated light pipe systems will be presented for comparison and to illustrate the effects of errors. Since it is impossible or impractical to eliminate all injection molding fabrication errors from manufactured light pipe systems, this paper will conclude with a discussion of the circumstances under which individual errors should be diminished or eliminated. INTRODUCTION BACKGROUND Plastic injection molded light pipes (or thick waveguides) offer an ideal nonimaging optical solution for automotive lighting and illumination. Figure 1 shows an example of a light pipe system for interior automotive display illumination such as radios and instrument clusters. Similar systems, typically of larger scale, are beginning to be used for general interior and exterior automotive lighting systems, including tail, puddle, and forward lighting applications.[1][2][3] Since image formation is not the goal in these applications, the refraction and total internal reflection (TIR) properties of light pipes can be efficiently used to collect, transport, and output light.[4] Light pipes can also be used to tailor the output light distribution to the needs of the illumation task.[5][6] Figure 1. Example interior automotive light pipe illumination system. Because they are injection molded, light pipes are well suited for automotive manufacturing. They are relatively inexpensive, easy to fabricate in large quantities (millions per year), and are versatile to design. However, the injection molding process can introduce some surface and volume properties that are detrimental to the optimal performance of these systems. Surface and volume errors introduced by the injection molding manufacturing process include: sinks, voids, knit and flow lines, warpage, and blushing. Of these the most common and serious molding errors are sinks and voids. Sinks occur in the injection molding process when a large volume of liquid plastic is allowed to cool to rapidly. This results in a sinking or dimpling of the surface as the cooling plastic shrinks away from the tool cavity walls. Sinks can range from a very slight surface deformation to a severe deformation that extends through the part. Voids, or included areas of air, occur in injection molding when the melt temperature or cavity pressure is insufficient to fully fill the mold. Voids can range from very small bubbles to relatively large cavities. Until recently the detrimental ramifications to lighting quality caused by injection molding was not readily apparent. However, with the advent of powerful commercial optical modeling software packages and new nonimaging design tools, light pipe system design has become more precise and efficient.[7][8][9] In fact, design precision has increased to such a level that injection molding fabrication effects are now becoming a limiting factor in some cases.